Bubble cap trays are well known and are extensively employed in distillation columns. A useful discussion of bubble cap trays may be found in Design of Equilibrium Stage Processes, Smith, McGraw-Hill Book Company (1963), Ch 14 Bolles, pp 474-538.
A bubble cap tray comprises a tray deck upon which is disposed a plurality of bubble caps disposed in rows. Liquid flows onto the tray from the tray above it via a downcomer, then flows under a downcomer baffle, usually over an inlet weir, and onto the tray deck. On the tray deck are situated bubble caps such that the vapor issuing from the bubble caps will contact the liquid flowing across the tray. The liquid flows between and/or over the bubble caps from the inlet weir to an outlet weir. The liquid is aerated by the vapor and takes on the form of a froth or foamy liquid. The mechanism of froth movement across the tray is that the froth height or static head of liquid at the tray inlet is higher than the froth height or static head of liquid at the tray outlet. This hydraulic gradient enables the liquid to flow towards the tray outlet. Once the froth reaches the tray outlet, it returns to its liquid form upon deaeration. The deaeration usually occurs in a downcomer on the downstream side of the outlet weir. Once in the downcomer the liquid proceeds down to the next tray and the process is repeated until the bottom of the column is reached. The vapor on a bubble cap tray comes from the tray below and passes up through the tray deck and into the bubble caps through risers. The vapor hits the inside top of the bubble cap and then flows down through the annular area between the outside of the riser and the inside of the bubble cap. The vapor enters the liquid by bubbling through openings in the cap or under the edge of the cap and escapes into the froth on the tray deck. Once on the tray deck the bubbles pass upward through the froth until the top of the froth is reached where the bubbles burst and the vapor continues upwards to the next tray where the process is repeated until the top of the column is reached. Mass transfer occurs between the liquid and vapor. The more volatile components preferentially pass into the vapor phase while the less volatile components pass into the liquid phase. This occurs on each tray causing the more volatile components to concentrate at the top of the column and the less volatile components to accumulate at the bottom of the column.
A problem with bubble cap trays is their limited turndown capability although their capability range is better than other mass transfer devices. Turndown is the ratio of the maximum flowrate of a phase to the minimum flowrate of that same phase while still achieving satisfactory mass transfer. The turndown for conventional bubble cap trays is about 5:1. Thus conventional bubble cap trays cannot be advantageously employed in a distillation column for the separation of feed at large capacity variations. One such separation is the separation of nitrogen from methane in a nitrogen rejection unit which processes a feed from a natural gas reservoir which has been injected with nitrogen as part of an enhanced recovery operation. These units are required to process a very wide range of vapor and liquid flowrates as the nitrogen content of the feed to the unit increases from about 5 to about 80 mole percent during the period of operation of the plant. This application may require a turndown of 20:1 or even higher.
It is therefore an object of this invention to provide an improved bubble cap tray which can operate efficiently at higher turndown than is possible with conventional bubble cap trays.